1. Field of the Invention
The present invention relates to a two-wire load control device, specifically a two-wire smart dimmer having a microprocessor and a power supply for generating a direct-current (DC) voltage for powering the microprocessor.
2. Description of the Related Art
A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus to control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.
Smart wall-mounted dimmers may include a user interface typically having a plurality of buttons for receiving inputs from a user and a plurality of status indicators for providing feedback to the user. These smart dimmers typically include a microprocessor or other processing device for allowing an advanced set of control features and feedback options to the end user. An example of a smart dimmer is disclosed in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety.
A simplified block diagram of a prior art two-wire dimmer 100 is shown in FIG. 1. The dimmer 100 has a hot terminal 102 connected to an AC voltage source 104 and a dimmed hot terminal 106 connected to a lighting load 108. The dimmer 100 employs a semiconductor switch 110 coupled between the hot terminal 102 and the dimmed hot terminal 106, to control the current through, and thus the intensity of, the lighting load 108. The semiconductor switch 110 has a control input (or gate), which is connected to a gate drive circuit 112. The input to the gate will render the semiconductor switch 110 conductive or non-conductive, which in turn controls the power supplied to the lighting load 108. The gate drive circuit 112 provides control inputs to the semiconductor switch 110 in response to command signals from a microprocessor 114.
The microprocessor 114 receives user inputs from a plurality of buttons 116 and generates command signals to drive a plurality of light emitting diodes (LEDs) 118 for visual feedback to the user of the dimmer 100. A zero-crossing detect circuit 120 determines the zero-crossing points of the AC source voltage from the AC power supply 104. A zero-crossing is defined as the time at which the AC supply voltage transitions from positive to negative polarity, or from negative to positive polarity, at the beginning of each half-cycle. The zero-crossing information is provided as an input to the microprocessor 114. The microprocessor 114 generates the gate control signals to operate the semiconductor switch 110 to thus provide voltage from the AC power supply 104 to the lighting load 108 at predetermined times relative to the zero-crossing points of the AC waveform.
In order to provide a DC voltage VCC to power the microprocessor 114 and other low-voltage circuitry, the dimmer 100 includes a cat-ear power supply 122. A cat-ear power supply draws current only near the zero-crossings of the AC source voltage and derives its name from the shape of the current waveform that it draws from the AC voltage source. Because the dimmer 100 only has two terminals 102, 106 (i.e., it is a two-wire dimmer), the power supply 122 must draw current through the connected lighting load 108. In order for the power supply 122 to be able to draw sufficient current, the semiconductor switch 110 must be non-conductive so that a sufficient voltage is available across the power supply. Thus, the semiconductor 110 cannot be turned on for the entire length of a half-cycle, even when the maximum voltage across the lighting load 108 is desired.
A simplified schematic diagram of the prior art cat-ear power supply 122 is shown in FIG. 2. The cat-ear power supply is provided on the DC-side of a bridge rectifier comprising diodes D202, D204, D206, D208, such that the cat-ear power supply is able to generate the DC voltage VCC. The DC voltage VCC is produced across an energy storage capacitor C210 and has a magnitude that is appropriate to power the microprocessor 114 and other low-voltage circuitry (e.g., approximately 5VDC). The side of the energy storage capacitor C210 that is connected to circuit common (i.e., the cathode) is also connected to an NPN transistor Q212 and a PNP transistor Q214. A zener diode Z216 and a diode D218 are provided in series between the DC voltage VCC and the base of the transistor Q214. The forward voltage drop of the diode D218 is approximately the same as the emitter-base voltage of the transistor Q214. Accordingly, the magnitude of the DC voltage VCC produced across the energy storage capacitor C210 is limited to approximately the same magnitude as the break-over voltage of the zener diode Z216, e.g., 5.1 volts.
The primary charging or energy-receiving circuit for the energy storage capacitor C210 is through the transistor Q212 and a current limiting resistor R220. When transistor Q214 is conductive, a voltage is produced across a resistor R222, and thus the base-emitter junction of the transistor Q212, causing the transistor Q212 to conduct. A resistor R224 maintains the base current needed to keep the transistor Q214 conductive.
When the voltage across the power supply 122 reaches a certain magnitude, a PNP transistor Q226 begins to conduct, causing the transistor Q214, and thus the transistor Q212, to stop conducting. A zener diode Z228 and a resistor R230 are connected in series between the base of the transistor Q226 and the emitter of the transistor Q212. A resistor R232 is connected across the base-emitter junction of the transistor Q226. The zener diode Z228 will begin to conduct when the voltage at the base of the transistor Q226 exceeds the break-over voltage of the zener diode (approximately 12V). When the voltage across the resistor R232 exceeds the required emitter-base voltage of the transistor Q226, the transistor Q226 will begin to conduct. Thus, when an appropriate voltage (e.g., approximately 16V) is produced across the power supply 122, the transistor Q226 will begin to conduct, causing the transistors Q212, Q214 to stop conducting, thus halting the charging of the energy storage capacitor C210. A capacitor C234 is coupled across the resistor R232 to provide a time delay in the shut-off of the charging of the energy storage capacitor C210. When the voltage across the power supply 122 drops below the appropriate level (e.g., approximately 16V), the transistor Q226 stops conducting and the energy storage capacitor C210 is able to charge again.
The prior art cat-ear power supply 122 has some disadvantages. First, the period of time that the energy storage capacitor C210 is able to charge each half-cycle is set by the values of the chosen components of the power supply 122. If the power supply 122 is connected to an AC voltage source when the capacitor C210 is uncharged, the power supply is susceptible to drawing the initial charging current at the peak of the AC voltage, which can produce a very large current in the charging circuit of the power supply 122, especially through the transistor Q212 and the resistor 8220. To prevent these parts from being damaged under this condition, the transistor Q212 and resistor R220 must be physically larger, more costly parts than would be required if only operating under normal conditions.
To ensure that the power supply 122 is able to draw enough current to maintain its output voltage at all times, the semiconductor switch 110 is turned off for at least a minimum off-time each half-cycle. The proper operation of the dimmer 100 is constrained by a number of worst-case operating conditions, such as high current draw by the low-voltage circuitry, worst-case line voltage input (i.e. when the AC power supply voltage is lower than normal), and worst-case load conditions (such as the number and the wattage of the lamps, the type of the lamps, and variations in the operating characteristics of the lamps). The wattage of the lighting load 108 is particularly important since the AC voltage source 104 is coupled across the power supply 122 and the lighting load in series, and thus, the impedance of the lighting load directly affects the voltage developed across the power supply and the time required to charge the power supply. The impedance of a lighting load will decrease as the rated wattage is increased, and vice versa. Thus, the worst-case time required to charge the power supply 122 occurs when a low-wattage lamp is connected to the dimmer 100 since the impedance of the load will be substantially higher and the voltage across the power supply will be substantially lower with this type of load. When considering the worst-case conditions, 40 W lamps are often used as the minimum load likely to be encountered.
By considering these worst-case conditions, the minimum off-time is determined by calculating the off-time that will guarantee that the power supply 122 will charge fully for even the worst-case conditions. The resulting off-time generally ends up being a significant portion of each half-cycle and constrains the maximum light level of the attached lighting load 108. However, these worst-case conditions are often not encountered in practice. Under typical conditions, the semiconductor switch could be rendered conductive for a greater amount of time during each half-cycle in order to conduct current to the load for a greater amount of time. Accordingly, the lighting load 108 will reach a higher intensity that is closer to the intensity achieved when the full line voltage is provided to the load.
Some prior art dimmers have held the minimum off-time constant under all conditions, and thus have suffered from a smaller dimming range than would otherwise be possible. Another prior art two-wire dimmer 300, which is shown in FIG. 3, monitors the internal power supply and decreases conduction time of the semiconductor switch, if needed. The two-wire dimmer 300 is able to provide the maximum possible light intensity at high-end while simultaneously ensuring sufficient charging time for proper operation of an internal power supply, and hence, the dimmer. The dimmer 300 is described in greater detail in co-pending U.S. patent application Ser. No. 11/127,864, filed May 12, 2005, entitled DIMMER HAVING A POWER SUPPLY MONITORING CIRCUIT, which is incorporated herein by reference in its entirety.
Referring to FIG. 3, the two-wire dimmer 300 has two connections: a hot terminal 302 to an AC power supply 304 and a dimmed hot terminal 306 to a lighting load 308. To control the AC voltage delivered to the lighting load 308, two field-effect transistors (FETs) 310A, 310B are provided in anti-serial connection between the hot terminal 302 and the dimmed hot terminal 306. The first FET 310A conducts during the positive half-cycle of the AC waveform and the second FET 310B conducts during the negative half-cycle of the AC waveform. The conduction state of the FETs 310A, 310B is determined by a microprocessor 314 that interfaces to the FETs through a gate drive circuit 312. The dimmer 300 also includes a plurality of buttons 316 for input from a user and a plurality of LEDs 318 for visual feedback to the user. The microprocessor 314 determines the appropriate dimming level of the lighting load 308 from the inputs from the buttons 316. A zero-crossing detect circuit 320 receives the AC supply voltage through diode 321A in the positive half-cycles and through diode 321B in the negative half-cycles and provides a control signal to the microprocessor 314 that identifies the zero-crossings of the AC supply voltage.
The dimmer 300 further includes a power supply 322 to power the microprocessor 314 and the other low-voltage circuitry. The power supply 322 is only able to charge when the FETs 310A, 310B are both turned off (i.e., they are non-conducting) and there is a sufficient voltage potential across the dimmer. The power supply 322 is coupled to an input capacitor 324 and an output capacitor 326. The output capacitor 326 holds the output of the power supply VCC at a substantially constant DC voltage to provide power for the microprocessor 314. The input of the power supply 322 is coupled to the hot terminal 302 and the dimmed hot terminal 306 through the two diodes 321A, 321B, such that the input capacitor 324 charges during both the positive and negative half-cycles.
The dimmer 300 also includes a voltage divider that comprises two resistors 328, 330 and is coupled between the input of the power supply 322 and circuit common. The voltage divider produces a sense voltage VS at the junction of the two resistors 328, 330. The sense voltage VS is provided to the microprocessor 314 to monitor the voltage level at the input of the power supply 322. The microprocessor 314 preferably includes an analog-to-digital converter (ADC) for sampling the value of the sense voltage VS. The microprocessor 314 monitors the sense voltage VS and decreases the conduction times of the FETs 310A, 310B when the sense voltage VS drops below a first predetermined voltage threshold V1. Further, the microprocessor 314 increases the conduction times of the FETs 310A, 310B when the sense voltage then rises above a second predetermined voltage threshold V2, greater than the first threshold V1. Alternatively, if the microprocessor does not include an ADC, the dimmer 100 could include a hardware comparison circuit, including one or more comparator integrated circuits, to compare the sense voltage with the first and second voltage thresholds and then provide a logic signal to the microprocessor 314.
By monitoring the input of the power supply 322, the microprocessor 314 of the dimmer 300 is able to determine when the input voltage has dropped to a level that is inappropriate for continued charging of the input capacitor 324. For example, if the sense voltage VS falls below the first voltage threshold V1, then the capacitor 324 needs a greater time to properly charge and the on-times of the FETs 310A, 310B are decreased. On the other hand, if the sense voltage VS remains above the first voltage threshold V1, the input capacitor 324 is able to properly charge each half-cycle.
Thus, the microprocessor 314 continuously monitors the voltage on the input capacitor 324 and automatically decreases the conduction times of the FETs 310A, 310B when the voltage falls to a level that will not guarantee proper operation of the power supply 322. The dimmer 300 is able to provide the maximum possible conduction times of the FETs 310A, 310B at high end (i.e., maximum light intensity) while simultaneously ensuring sufficient charging time for proper operation of the power supply 322.
However, the dimmer 300 of FIG. 3 requires that the microprocessor 314 include an ADC or that a hardware comparison circuit be included between the power supply 322 and the microprocessor. Also, the dimmer 300 is not able to control the power supply 322 directly, but operates the FETs 310A, 310B in order to indirectly control the time during which the power supply draws current.
Thus, there exists a need for a simple cat-ear power supply for a dimmer that is operable to be monitored and directly controlled by a microprocessor, specifically to control the time period that the power supply draws current and to control the conduction time of the semiconductor switch in response to the operation of the power supply, without the need for an ADC or a complex hardware comparison circuit.